We have created a global template that you can use to monitor the list of subnets maintained by your DHCP server. Go to Network > Device Templates > Global Templates and search for the template named DHCP Server. You can import and edit the device template to add more metrics of your choice using Custom Performance counters.
It will also be good if DNS server monitoring using WMI also be in the road-map. The current DNS server monitoring using site24x7 is very basic. It does not provide any insight other than availability and response time. There are so much else to monitor in DNS server.